Serial ATA Simplified

Most people, when purchasing a new computer, pay careful attention to how
fast the CPU is and how much memory it has. A third consideration is how
large the hard drive is. Unfortunately, people rarely ask how fast the
drive is. Just like a chain is only as strong as its weakest link, a computer
is only as fast as its slowest component. These days, the hard drive is the
slowest commonly used component in most computers. Parallel Advanced
Technology Attachment / Integrated Drive Electronics (PATA / IDE) hard disks
are the most common style to have in a home computer. IDE is also 20 years
old, has crippling limitations, and will very soon be replaced by a new
technology called Serial ATA (SATA).

The general rule with buying a hard disk for a new personal computer used to
be 'Find a hard disk that you don't think you'll be able to fill,
then buy one that's four times that size.' This rule would typically
allow you to have one disk for the life of that new computer. Unfortunately,
with the dramatic increase in software size, and broadband connections
offering the ability to download fantastic amounts of data, the rule has
become 'Buy the largest disk you can possibly afford, and buy another one
next year'. The economy of this form of planned obsolescence is
indisputably poor, but it has a further technical problem: drives are having
to hold more data than the computer can efficiently read from them.

Parallel Breakdown

The
Parallel IDE
bus invented nearly two decades ago has gone through several renovations over
the years to offer greater device compatibility (ATA 66/100/133), and greater
speeds, but many of its original limitations remain.

Two drive limit per channel (Master / Slave)

Shared bandwidth

Max. bus bandwidth of 133 Mbps

Strict cable length limits

Large connector wastes valuable drive real estate

Wide ribbon cable blocks air flow inside computers

High power requirements

No growth road map

To illustrate how bus speed affects computer performance, we can estimate the
time it would take to transfer all the data off a single drive connected to a
standard PATA bus.

Disk Size

Bus Speed

Transfer Time

8 GB

33 Mbps

~32 Min

20 GB

66 Mbps

~40 Min

60 GB

100 Mbps

~80 Min

250 GB

133 Mbps

~250 Min

These times are calculated by using the theoretical maximum bus bandwidth,
and we rarely get the full capacity of a bus from one drive. Furthermore, if
we're transferring to another device on the same channel, these bus
speeds would be cut in half, and the times would double.

To SATA and Beyond

Specifications for
Serial ATA
were introduced in August of 2001, and were developed specifically to address
limitations in our current ATA standards. The Serial ATA 1.0 specification
has a base bandwidth of 150Mbps and planned rates up to 10Gbps. Another speed
advantage that SATA has over parallel technologies is its interconnect. SATA
drives each connect directly to the controller, so bandwidth isn't shared
between devices on a chain. This has the side effect of reducing overhead in
device control commands and allows for added scalability. Instead of the
number of drives on a controller being determined by the bus architecture or
a block command set, the vendor can choose how many connectors to make
available on their host adapters.

Small Bytes of Serial

The public stigma that serial data transfer is slow has slowed the widespread
adoption of Serial ATA. Asking the general computer user where their
computer's serial port is located will typically yield some finger
pointing at a 9 pin connector on the back of the computer. Further questions
relating to their computer's parallel port will be answered with more
finger pointing at the 25 pin connector their printer used to be plugged in
to before they bought a USB printer. If you then asked this computer user to
choose if parallel or serial ports were faster, they would probably indicate
that parallel was faster.

In truth, computers contain a number of serial and parallel buses both inside
and out.
USB,
FireWire
(IEEE-1394),
PS/2,
RS-232
and even Ethernet use serial transfer protocols. PCI, AGP, IDE, Parallel
(IEEE-1284) and (the big one) the Front-Side bus are all
parallel interconnects.

As you may know, serial systems transfer one bit at every clock tick of their
controller, and parallel systems transfer several bits at every clock. The
number of bits a parallel bus transfers per clock is called it's
'width'. To increase the transfer speed of a bus, either the width or
the frequency of the clock ticks has to increase. To give an example, the
first SCSI systems were 8 bits in width, and Wide SCSI was 16 bit, doubling
the width doubled the transfer speed. Fast SCSI which came out about the same
time left the bus width at 8 bits, but doubled the clock frequency, also
doubling the transfer speed.

Breathing Room

Another advantage that Serial ATA offers over traditional PATA is a more
compact cable design. This becomes quite important as computers get smaller
and smaller. Airflow and cooling are always critical factors in computer
design. Compare the old PATA ribbon cable with a newer SATA cable, and the
differences become clear:

The narrow design of the Serial ATA cable allows for more flexibility in
running the cable, as well as improved airflow within the computer.

Infiltrating RAID

If SATA hard disks are going to compete against
SCSI
in the RAID arena, SATA must offer the same features as SCSI drives do in
high-availability systems, but at a greatly reduced cost. The failure rate
for ATA drives has always been greater than SCSI drives, so the Mean Time
Between Failure (MTBF) becomes the major difference when calculating the
total cost of ownership. SATA drives can be spun down and removed while the
system is running, so a failed RAID drive could be replaced just like current
SCSI disks.

Laptop Love

Connectors for SATA require only seven pins for data, and four for power.
This makes it possible for 2.5" hard disks to further reduce in size,
and the reduced power requirements (SATA is 0.5v where PATA is 5.0v) will
extend battery life for portables, and reduce thermal radiation. Also, having
the same connector on both laptop and desktop workstations will make pulling
data off damaged laptops easier. The SATA connector can also be used for
external drives.

I want to use SATA. What can I do?

Serial ATA hard disks and PCI controller cards have been available on the
market for about a year, but their use has been limited to niche markets
until recently. In the past few months several major computer vendors have
started offering machines using SATA technologies, and the price difference
between standard PATA drives and SATA drives has dropped significantly. If
you buy a new computer with SATA controllers, you can still use your existing
hard disk on those SATA controllers using adaptors. These can also be useful
for moving data from existing IDE drives to SATA disks.

Support for SATA drives is still a little sketchy a the moment. Windows
doesn't offer Mass Storage drivers for SATA by default, so you can't
install a fresh copy of Windows onto an SATA drive without the SATA
controller drivers on a floppy, and hitting F6 during the install. Linux
drivers are available, but they're not as mature as existing SCSI or IDE
drivers. Also, utilities such as hdparm haven't been fully updated for
SATA just yet.

Glossary

Bus
A computer bus is a set of wires that transfers power or data between
components. When referring to modern hard disk buses, we talk about
Integrated Drive Electronics (IDE) and Small Computer System Interface
(SCSI) buses. As SCSI systems are typically much more expensive, it's
reserved for the few situations where its speed and scalability features are
required (servers, video processing, etc.). Our current form of IDE and
most types of SCSI are parallel buses.

RAID
Redundant Array of Independent Disks. RAID comes in several forms, each with
their own benefits. RAID 0 (also known as striping) is the simplest form of
RAID, and simply distributes the usage of the drives for increased speed.
It's not considered a true RAID, because if one disk fails, the data is
lost, and the data on the other disks becomes hard to retrieve. RAID 5
requires a minimum of three disks, stores duplicate data on each of them,
and reads data from them simultaneously. If 1MB of sequential data is drawn
from a RAID 5 array of 4 disks, one quarter of the data is read from each
disk. Conversely, if data needs to be written to the array, it must be
written (at least partially) to all the disks. Unlike RAID 0, if one drive
fails in a RAID 5 array, it can simply be replaced and the RAID array will
automatically rebuild the data that's supposed to be on that disk. When
RAID was first marketed, the 'I' in the acronym stood for
'Inexpensive'. However, RAID arrays have become anything but
Inexpensive, and the term was changed.